A thin steel plate with high strength, which is called as high-tension steel, has been increasingly used for a structural member of an automobile. Such a high-tension steel is effective for weight reduction and safety improvement, but is heavier than a low specific gravity material such as aluminum. The high-tension steel has problems such as decrease of forming easiness, increase of a forming load, and decrease of dimensional accuracy due to its high strength. In order to solve these problems, in recent years, multiple-material approach has been adopted, in which extrusion molded products, cast products, or press molded products made of aluminum with a smaller specific gravity than that of steel plates is utilized together with steel parts.
In the multiple-material approach, joining of steel parts and aluminum parts involves a problem. In the welding technique typified by spot welding, a brittle intermetallic compound (IMC) is generated at the interface between a steel plate and an aluminum plate, so that practically applied joining techniques include electromagnetic forming bonding, screw fastening typified by bolts and nuts, friction stir welding (FSW), rivets, self-piercing rivets (SPR), mechanical clinching, and adhesion.
In press-fitting by electromagnetic forming, a solenoid formed coil is inserted into a pipe component fitted to a mating component, and followed by applying impact current to the coil so that induced current is induced in the conductive pipe by change in a magnetic field generated by applying impact current. Electromagnetic force is generated between magnetic field generated by the primary current of the coil and induced current flowing in the opposite direction along the circumferential direction of the pipe. This causes that the pipe receives outward moving force, and thus is enlarged and deformed to join the pipe to the mating component by press-fitting. This joining method is suitable for copper and aluminum, which are highly electrically conductive, and is practically used in parts of joining of automobile components.
JP 2007-284039 A discloses a press-fit joining technology by electromagnetic forming for multiple-material approach. In the technology of JP 2007-284039 A, a bumper reinforcement made of a metal material and having a hollow section is expanded and deformed by electromagnetic forming, and accordingly, fitted and joined to a hole provided to a bumper stay made of aluminum alloy.